Various flat display devices, such as LCD (Liquid Crystal Display Device), PDP (Plasma Display Panel), ELD (Electro Luminescent Display), VFD (Vacuum Fluorescent Display), are used as display devices in various apparatuses.
Of the various display devices replacing the CRT (Cathode Ray Tube), the LCD is often used for mobile display devices owing to advantages of good picture quality, light weight, thin profile, and low power consumption. Besides mobile display devices, such as monitors for notebook computers, the LCD is developed in various forms as monitors for televisions for receiving and displaying a broadcasting signal, and monitors for computers.
For using the LCD as general display devices, one aspect of the of development of the LCD focuses on realization of a high quality picture, such as high definition, high luminance, and large sized picture, while the features of light weight, thin profile, and low power consumption are maintained.
Referring to FIG. 1, the related art liquid crystal display device is provided with a first substrate 1 and a second substrate 2 bonded together with a space between the substrates, and a liquid crystal layer 3 injected between the first substrate 1 and the second substrate 2.
A plurality of gate lines 4 arranged on the first substrate at regular intervals in one direction, and a plurality of data lines 5 at regular intervals perpendicular to the gate lines 4 to define pixel regions ‘P’, wherein a pixel electrode 6 is formed on each of pixel regions ‘P’, and a plurality of thin film transistors ‘T’ are respectively formed at portions where the gate lines 4 and the data lines 5 cross, so as to be switched in response to a signal on the gate line 4 for transmission of a data signal from the data line 5 to each pixel electrode 6.
A black matrix layer 7 is formed on the second substrate 2 for shielding a light incident on portions excluding the pixel regions ‘P’, the R, G, G color filter layers 8 at portions opposite to the pixel regions respectively for expressing colors, and a common electrode 9 on the color filter layer 8.
The liquid crystal display device can display a picture as the liquid crystal layer 3 is oriented between the first and second substrates 1, and 2 by an electric field between the pixel electrode 6 and the common electrode 9, to regulate a quantity of a light transmit through the liquid crystal layer 3 depending on an extent of orientation of the liquid crystal layer 3.
This type of liquid crystal display device is called as a TN (Twisted Nematic) mode liquid crystal display device. Since the TN mode liquid crystal display device has a drawback of a narrow angle of view, in order to overcome such a drawback, an IPS (In-Plane Switching) mode liquid crystal device has been developed.
The IPS mode liquid crystal device has the pixel electrodes and the common electrodes arranged in parallel at regular intervals for forming an in-plane electric field between the pixel electrodes and the common electrodes, to orient the liquid crystal layer.
Spacers for maintaining a fixed gap of the liquid crystal layer are formed between the first, and second substrate 1, and 2 of the liquid crystal display device.
There are ball spacers and column spacers depending on shapes thereof. The ball spacers are spherical, scattered on the first, and second substrates 1, and 2, move freely even after bonding of the first, and second substrates 1, and 2, and have a smaller contact area with the first, and second substrates 1, and 2. The column spacers are columnar and fixed on the first, or second substrate 1, or 2 and formed in an array step. Accordingly, the column spacer has a greater contact area with the first, and second substrates 1, and 2 than the ball spacer.
The column spacer is mostly used in liquid crystal display devices fabricated by the liquid crystal dispensing method, because of the problems caused when the ball spacer is used in the liquid crystal dispensing method. In the liquid crystal dispending, the ball spacers are scattered before or after dispensing of the liquid crystals and bonding of the first and second substrates. If the spacers are scattered after dispensing of the liquid crystals, the liquid crystal impedes uniform spread of the spacers to proper positions, and if the spacers are scattered before dispensing of the liquid crystals, even if the spacers are scattered uniformly, the dispensing of the liquid crystals disturbs the uniform distribution of the spacers. Moreover, in the latter case, since the liquid crystals are dispensed on a non-uniform spacer scattered surface, fluidity of the liquid crystals is poor. Therefore, in the liquid crystal display device fabricated by the liquid crystal dispensing method, the column spacers are used for maintaining a cell gap between two substrates.
FIG. 2 illustrates a section of a related art IPS mode liquid crystal display device. An IPS mode liquid crystal display device is provided with a thin film transistor array substrate (TFT substrate) 30 having a thin film transistor array formed thereon, a color filter array substrate (CF substrate) 40 having a color filter array formed thereon, a liquid crystal layer 50 filled between the TFT substrate 30 and the CF substrate 40, and column spacers 45 between the TFT substrate 30 and the CF substrate 40.
Although not shown, the TFT substrate 30 includes first substrate having gate lines and data lines arranged thereon perpendicular to each other to define pixel regions, a plurality of TFT respectively formed at portions where the gate lines and the data lines cross, a pixel electrode, and a common electrode formed alternately on each of the pixel regions. Between layers of the gate line and the data line, there is a gate insulating film, and between layers of the data line and the pixel electrode, there is a protective film.
The CF substrate 40 opposite to the TFT substrate 30 includes a second substrate 41, a black matrix layer 42 for covering a non-pixel region (gate lines, data lines, and TFT regions), a color filter layer 43 having R, G, or B pigment formed in a sequence on the second substrate 41, and disposed opposite to the pixel regions, and an overcoat layer 44 formed on an entire surface of the second substrate 41 including the black matrix layer 42 and the color filter layer 43.
On a predetermined region of the overcoat layer 44, the column spacers 45 are formed and an alignment film 46 is formed on a surface of the CF substrate 40 including the column spacers 45 in a cell forming step after an array forming step. Though not shown, an alignment film is also formed on the TFT substrate 30 in an initial step of the cell forming step.
The (IPS mode) liquid crystal display device having column spacers exhibits a luminance non-uniformity in a black state.
Referring to FIG. 3A, if a finger touches subsequently, and sweeps a surface of the TFT substrate 30 or the CF substrate 40] of the liquid crystal panel 10 in a direction, the CF substrate 45 of the liquid crystal panel shifts a distance in the direction the finger sweeps as shown in FIG. 3B. In this instance, however, the liquid crystals 50 between the column spacers 45 fail to return to an original state, such that a touched portion is left as a milky-white spot on the liquid crystal panel 10. Moreover, as shown in FIG. 3B, a swept portion by the finger has less of the liquid crystals 50 while the liquid crystals gather at a final contact portion to form a protruded shape. In this instance, the protruded portion with the liquid crystal gathered thereon has a cell gap h1 higher than a cell gap h2 of other portion that is defined by a height of the column spacer 45, Accordingly, a problem occurs in which a touched portion has a non-uniform luminance compared to other portions, which may cause a leakage of light.
This problem is caused by high friction between an upper surface of the column spacer 45 formed on the CF substrate and the TFT substrate 30. That is, after the CF substrate shifts with respect to the TFT substrate 30, the CF substrate fails to return to an original position immediately and even if the finger is removed from the CF substrate, the leakage of light continues.
The force with which the column spacer 45 in close contact with the TFT substrate 30 pulls the TFT substrate 30 is greater than a force with which the CF substrate tends to return to an original position. In another aspect, the shrinkage or slackening of a polarization plate attached to the liquid crystal panel by change of environmental humidity and temperature pulls the substrate in a direction of deformation to bend the substrate, which disturbs the orientation of the liquid crystals.
Since the touch-related problem can occur by contact, such as wiping of the panel surface of the liquid crystal display device, the non-uniformity of luminance in a black state caused by the touch can occur during fabrication or use of the liquid crystal display device. Moreover, there is a tendency for the non-uniformity of luminance in a black state caused by the touch becomes worse as a size of screen becomes larger. When the size of the screen is large, control of a liquid crystal amount is difficult.
FIG. 4A illustrates a diagram showing a bent state of a substrate before attachment of the polarization plate, and FIG. 4B illustrates a diagram showing a bent state of a substrate after attachment of the polarization plate. It can be seen that the bending tends to be more intense at edges rather than at a center of the liquid crystal panel. This is because of a difference of thermal expansion coefficients between the substrate and the polarization plate causes a difference of lengths between the polarization plate of a film and the substrate of a glass plate arising from shrinkage and expansion of the polarization plate and the substrate at the time of thermal fabrication process (heating and cooling down) after the attachment of the polarization plate.
FIG. 5 illustrates a flow chart showing the steps of a related art method for fabricating a color filter array in a liquid crystal display device. The method includes the steps of: providing a second substrate (S10); forming a black matrix layer (S11); forming a R, G, B color filter layer on the second substrate including the black matrix layer. In this instance, respective color films are formed by dividing and patterning regions of the color films (S12). An overcoat layer is formed on the second substrate including the R, G, B color filter layer (S13), and, an organic film is coated on an entire surface of the overcoat layer. The organic film is removed selectively to form column spacers (S14).
A TFT array substrate suitable to be disposed opposite to the color filter substrate is formed according to the following steps: a metal is deposited on a first substrate, and removed selectively, to form a gate line having a gate electrode projected therefrom; a common line is formed in a same direction as the gate line by patterning the metal, and a common electrode is formed branched from the common line in a pixel region. Then, a gate insulating film is formed on an entire surface of the first substrate to cover the gate line having the gate electrode projected therefrom; a semiconductor layer is deposited on the gate insulating film, and removed selectively, to form a semiconductor layer on the gate insulating film over the gate electrode. A metal is deposited on the gate insulating film including the semiconductor layer, and removed selectively, to form a data line perpendicular to the gate line having a source electrode projected therefrom, and a drain electrode spaced a predetermined distance from the source electrode. Then, a protective film is formed on the gate insulating film including the data line and the source/drain electrodes. The protective film is removed selectively, to form a contact hole exposing a predetermined portion of the drain electrode, and a transparent electrode is deposited on an entire surface of the protective film including the contact hole, and removed selectively, to form a pixel electrode electrically connected to the drain electrode and alternating with the common electrode.
A process for fabricating a cell in the related art liquid crystal display device will be described with reference to FIG. 6. A first substrate having a plurality of thin film transistors defined thereon, and a second substrate having a color filter array defined thereon are provided. The thin film transistor array, and the color filter array are formed on a display region of each unit panel.
Then, an alignment film is formed on the display region of each unit panel of the first substrate and the second substrate (S21). Liquid crystals are dispensed on one of the first, and second substrates (S22). A seal pattern having a predetermined width is formed on a non-display region (a portion excluding the active portion) of one of the first, and second substrates (S23).
The other substrate having no liquid crystal dispensed thereon is inverted (S24), and the first and the second substrates are bonded to form a liquid crystal panel (S25). Though the flow chart illustrates that the liquid crystals are dispensed on the first substrate, and the seal pattern is formed on the second substrate, the liquid crystals may be dispensed on the second substrate, and the seal pattern may be formed on the first substrate. Alternatively, both the dispensing of the liquid crystals and the formation of the seal pattern may be made only on one of the substrates. In any case, the liquid crystal will not be dispensed on the substrate that will be inverted.
An ultra-violet (UV) beam is directed to the first substrate through the seal pattern, or to the seal pattern on a back side of the second substrate, to cure the seal pattern, to bond the first, and second substrates together (S26). Then, the liquid crystal panel is cut/processed in unit of the unit panel, to form a plurality of unit liquid crystal panels (S27).
A pad portion (a portion of the first substrate formed greater than the second substrate, so as to be exposed after the bonding) of the unit liquid crystal panel is inspected to determine whether the unit liquid crystal panel is accepted, or rejected (S28).
However, the related art liquid crystal display device and a method for fabricating the same have the following problems. First, the liquid crystal display device involved in shrinkage and expansion of the polarization film after attachment of the polarization film to the surface of the panel, to cause bending of the liquid crystal panel coming from a difference of thermal expansion coefficients of the substrate and the polarization film. The bending becomes the more extensive as a size of the panel becomes the greater. Second, the liquid crystal display device shows non-uniformity of luminance in a black state when the panel is touched, such as wiping of the panel.
The non-uniformity of luminance occurs in a black state because the shrinkage or slackening of a polarization plate attached to the liquid crystal panel by change of environmental humidity and temperature distorts the substrate by bending, which disturbs the orientation of the liquid crystals. Rubbing causes a translational shift in a range of 20˜100 μm between upper and lower substrates, to cause the leakage of light as the two substrates fail to return to original positions immediately even if the finger is removed. This is because a force with which a column spacer in close contact with an opposite substrate pulls the opposite substrate is greater than a force tending to return the substrate to an original position.